Electron emitting cathodes



No; a, 1966 A. T. WATTS ETAL ELECTRON EMITTING GATHODES Filed April 1, 1964 nw /vro/as ALBERT Z l f/Arrs a? THOMAS W Mmmp 5 Y A TTORNEY United States Patent 3,284,656 ELECTRON EMITTING CATHODES Albert Trevor Watts and Thomas William Myford, London, England, assignors to Thorn-AEI Radio Valves and Tubes Limited, London, England, a British com- Filed Apr. 1, 1964, Ser. No. 376,605 5 Claims. (Cl. 313-337) The present invention relates to electron-emitting cathodes.

In diodes for certain purposes, such as, for example, for use as television line-scan boosters (also known as efliciency diodes), it is necessary to provide insulation between the heater and the cathode sufiicient to withstand pulses of several thousand volts, the cathode being driven positive relatively to the heater. If this insulation is provided by thickening the normal heater insulating material, normally sintered alumina powder, the time between switching on the heater current and the diode becoming conducting is excessive.

It is well known that a substantial cause of delay in heat transfer from the heater to the cathode is the time lag caused by the thermal capacity of the heater itself, including the insulant, and that this time lag is very sensitive to the amount of insulant and its thermal contact with the heater.

One of the objects of the present invention is to provide an improved indirectly-heated electron-emitting cathode in which the insulation is adequate for the purpose above set forth and the time required for heating up is substantially shorter than with known diodes having the same insulating properties.

Another object of the present invention is to provide an improved electron-emitting directly-heated cathode which has a short heating-up time.

According to the present invention, in one aspect, there is provided an indirectly-heated electron-emitting cathode comprising a metal tube coated on its outside with electron-emissive material and mounted coaxially within the tube and spaced therefrom a heater in the form of a sleeve of braided refractory-metal wire mesh supported upon a core passing through the sleeve, connecting means being provided whereby heating current can be passed between the ends of the sleeve.

According to the present invention in another aspect there is provided an electron-emitting cathode comprising a sleeve of braided refractory metal wire mesh supported upon a core passing through the sleeve, connecting means being provided whereby heating current can be passed between the ends of the sleeve. The sleeve is preferably spaced from the core excepting at or near the two ends thereof.

Although the core may be of insulating material, for instance ceramic, it is preferably of refractory metal wire insulated from the sleeve at at least one end. The other end of the sleeve may be electrically connected to the core wire which then serves for making electrical connection with one end of the sleeve.

The sleeve may have connecting means in the form of metal eyelets fixed at each end thereof, the core passing through the eyelets.

When the heater is used in an indirectly-heated cathode in an efliciency diode, the cathode may be constituted by a cylindrical electron-emitting member coaxial with and spaced from the heater. In this case vacuum spacing is provided between the heater and the cathode and hence breakdowns, should they occur, tend to be self-healing. Moreover the provision between the heater and the cathode of an annular space of uniform radial thickness and the provision of smooth surfaces facing one another on 3,284,656 Patented Nov. 8, 1966 the heater and cathode will reduce the possibility of arcing.

When used in a directly-heated cathode, the sleeve of the heater according to the invention may be coated with electron-emitting material. Such a cathode has a very short heating-up time.

In a modified form of heater according to the invention, having, however, a longer heating-up time, the sleeve of refractory metal wire is braided directly upon acore of insulating material, such as ceramic, or of wire coated with insulating material.

The invention-will be describe-d, by way of example, with reference to the accompanying drawing in which:

FIG. 1 is a view in perspective, partly cut away, of an indirectly heated cathode according to the invention and FIG. 2 is a view in section, to a reduced scale, of the heater in FIG. 1.

In the drawing there is shown a cylindrical cathode 10 coated externally with electron-emitting material and mounted at one end upon a support 11, the cathode having within it a heater 12 in the form of a sleeve of braided tungsten wire. The sleeve 12 has an eyelet 13, 14 at each end. Through the eyelets is threaded a molybdenum wire 15 coated with insulating material 16, such as sintered aluminium oxide. The spaces 17, 18 (FIG. 2) between the eyelets 13, 14 and the core 15, 16 are then filled, as will be described later, in order to fix the heater to the core. A tag 19 is provided to connect the eyelet 13 to the wire 15 which forms one terminal for the feeding of heater current to the heater and a tag 20 connects with the eyelet 14 and provides the other feed terminal for the heater current.

The construction of the heater may be carried out as follows: In order to facilitate handling of the braided wire sleeve 12 it is coated with a thin layer of a stiffening material which can be subsequently removed. A suitable material is nitro-cellulose which can be removed by firing in moist hydrogen. The nickel eyelets 13 and 14 are inserted into the ends of the sleeve 12 and are then expanded to engage within the sleeve and so be temporarily fixed in position. A suspension of fine nickel powder is then painted over the tungsten wire braid where it is in contact with the eyelets and the nickel powder is sintered to the eyelets and the braid by heating in slightly moist hydrogen, or cracked ammonia gas, at between 875 C. and 950 C. for a time sufficient to give adequate sintering. In one example a suitable time was ten minutes. The humidity in the furnace ensures that any carbon formed by breakdown of the cellulose binders is removed by oxidation.

The heater unit comprising the sleeve with its eyelets is then passed over the core structure 15, 16, which has a length a little greater than that of the heater unit and the spaces 17 and 18 are filled with a thin suspension of aluminium oxide in a suitable vehicle such as nitro-cellulose solution. The tags 19 and 20 are then connected as already described.

The cathode shown in FIG. 1 may be assembled in a diode or other electron discharge device in the usual way.

What is claimed is:

1. A heater-cathode assembly suitable for use in an electron discharge device comprising in combination:

a cathode in the form of a metal tube having a coating of potentially electron-emissive material affixed to the outer surface thereof and a heater disposed within and spaced from said tube, said heater including a metal wire core extending along the longitudinal axis of said tube, a layer of insulating material afiixed to said wire core, a pair of metal eyelets affixed to said wire core, said eyelets being longitudinally spaced on said Wire core and said wire core passing through said eyelets, and a refractory metal Wire mesh sleeve having opposite ends thereof aflixed to said spaced eyelets, said wire mesh sleeve being supported by said eyelets afiixed to said wire core and having the greater part of the length thereof spaced from said layer affixed to and surrounding said Wire core, a pair 20 JOHN W. HUCKERT, Primary Examiner.

of metal eyelets insulatingly aflixed to said wire core, said eyelets being spaced along the longitudinal axis of said wire core and said wire core passing through said eyelets, a refractory metal wire mesh sleeve having opposite ends connected to said pair of eyelets, said sleeve being supported by said eyelets and having the greater part of the length thereof spaced from said insulating layer aflixed to said Wire core, and a coating of potentially electron-emissive material afiixed to the outer surface of said sleeve.

5. The heater-cathode assembly of claim 4 wherein one of said pair of eyelets is electrically connected to said wire core.

References Cited by the Examiner UNITED STATES PATENTS Bruckel et al. 313-340 X 1,730,837 10/1929 Armstrong 313340 1,869,567 8/1932 Krahl 313337 X 2,973,449 2/1961 Kuipers et al 313-340 X 3,191,089 6/1965 Farrow 313270 A. J. JAMES, Assistant Examiner. 

1. A HEATER-CATHODE ASSEMBLY SUITABLE FOR USE IN AN ELECTRON DISCHARGE COMPRISING IN COMBINATION: A CATHODE IN THE FORM OF A METAL TUBE HAVING A COATING OF POTENTIALLY ELECTRON-EMISSIVE MATERIAL AFFIXED TO THE OUTER SURFACE THEREOF AND A HEATER DISPOSED WITHIN AND SPACED FROM SAID TUBE, SAID HEATER INCLUDING A METAL WIRE CORE EXTENDING ALONG THE LONGITUDINDAL AXIS OF SAID TUBE, A LAYER OF INSULATING MATERIAL AFFIXED TO SAID WIRE CORE, A PAIR OF METAL EYELETS AFFIXED TO SAID WIRE CORE, SAID EYELETS BEING LONGITUDINALLY SPACED ON SAID WIRE CORE AND SAID WIRE CORE PASSING THROUGH SAID EYELETS, AND A REFRATORY METAL WIRE MESH SLEEVE HAVING OPPOSITE ENDS THEREOF AFFIXED TO SAID SPACED EYELETS, SAID WIRE MESH SLEEVE BEING SUPPORTED BY SAID EYELETS AFFIXED TO SAID WIRE CORE AND HAVING THE GREATER PART OF THE LENGTH THEREOF SPACED FROM SAID LAYER OF INSULATING MATERIAL AFFIXED TO SAID WIRE CORE. 